Explanation:
Why would Mars appear to move backwards?
Most of the time, the apparent motion of
Mars in
Earth's sky is in one direction,
slow but steady in front of the far distant stars.
About every two years, however, the
Earth passes Mars
as they orbit around the Sun.
During the most recent such pass in August,
Mars loomed particularly
large and bright.
Also during this time,
Mars appeared to move backwards in the sky,
a phenomenon called
retrograde motion.
Pictured above is a series of images digitally stacked
so that all of the stars images coincide.
Here, Mars appears to
trace out a loop in the sky.
At the top of the loop, Earth passed Mars and the
retrograde motion was the highest.
Retrograde motion can also be seen for other
Solar System planets.
In fact, by coincidence, the dotted line to the
right of the image center is Uranus
doing the same thing.